Improved in-vitro assays for chemical transformation of fibroblastic and epithelial cells was developed and co-carcinogenesis was used for the isolation of new transforming type-C viruses. Specifically: (1) Non-transformed, syngeneic epithelial and fibroblastic mouse embryo cell lines susceptible to chemical carcinogenesis in-vitro were established. Assay systems were developed to allow processing large numbers of carcinogen treated cells and performance of transformation experiments in a shorter period to time. (2) Co-carcinogenesis was used for the isolation of efficiently transforming type-C viruses in culture. (3) Growth promoting factors and transforming factors from chemically transformed epithelial cells were isolated by the use of nontransformed epithelial cells as test cells.